Target detection and identification using an imaging system that includes a camera is known in the art. Cameras, as known in the art, may be mounted on a plurality of objects, for example, closed circuit stationary cameras mounted on walls, geostationary satellites, portable cameras, that may also be mounted on a tripod, and moving platforms, including vehicles. Such a camera often requires a high level of sensitivity to light for use in poor visibility conditions. Also, a long focal lens is commonly employed to achieve high optical magnification. In conditions of poor visibility, for example at night, the low intensity of light reflected from a target, received by a camera used in an imaging system, results in low quality image resolution. In a case of low quality image resolution, such a camera cannot produce an image with an adequate signal-to-noise ratio to exploit the total resolution capability of the camera, and to discern fine details of an imaged target for identification purposes. Therefore, when imaging during night or in poor visibility conditions, such cameras require an auxiliary light source to illuminate a target and thereby improve the quality of the captured image. The auxiliary light source may be a laser device capable of producing a light beam that is parallel to the line-of-sight (LOS) of the camera, and that illuminates the field-of-view (FOV) of the camera or a part thereof. It is noted that television systems, in general, use a similar illumination method for adequate imaging. Also, long focal lenses, in general, have a limited light collecting capability due to their high f number. A high f number reduces the capability of a lens to collect enough photons to generate an adequate image, as compared to lenses with small f numbers.
An inherent problem in optical observation systems is the effect inclement weather conditions, such as humidity, haze, fog, mist, smoke and rain, have on the image produced. Particles or substances in the atmosphere may be associated with certain weather conditions. For example, haze results from aerosols in the air. These atmospheric particles or substances may obstruct the area between an observation system and a target to be observed. A similar case may result when an observation system operates in media other than air. For example, in underwater observations, the scattering of water particles, or of air particles above the water, may obstruct the area between an observation system and a target to be observed. In an observation system integrated with a laser device for target illumination, the interference of particles or substances in the medium between a system and a target can cause backscatter of the light beam. This is especially true when an auxiliary light source is used to illuminate a target at night, particularly if the illuminating source is located near the camera. The backscatter of the light beam results in blinding of a camera used in an observation system, especially if the camera has a high level of sensitivity, like an Intensified CCD (ICCD). The blinding of the camera reduces the contrast of an imaged target relative to the background. This blinding of the camera is referred to as self-blinding because it is partly caused by the observation system itself. During night conditions, contrast reduction significantly lowers the achievable range of imaging and target, or object, detection and identification, with respect to the attainable detection and identification range in daylight conditions.
In order to reduce the influence of particles or substances between an observation system and a target, and at night, in order to achieve longer identification ranges, the imaging sensor of a camera may need to be synchronized with respect to the time that the reflected light from the light illuminated target is due to be received by photodetectors located on the observation system. In particular, a laser generates short light pulses at a certain frequency. The imaging sensor of the camera is activated at the same frequency, but with a time delay that is related to the frequency. The light beam generated by the laser impinges on the target, and illuminates the target and the surrounding area. When the light beam is emitted toward the target, the receiving assembly of the imaging sensor of the camera is deactivated. A small part of the light is reflected from the target back towards the camera, which is activated as this reflected light reaches the camera.
Light which reflects off of particles or substances relatively close to the camera, in comparison to the longer distance between the camera and the target, will reach the receiving assembly of the camera while the camera is still deactivated. This light will therefore not be received by the camera and will not affect the contrast of the image. However, reflected light from the target and its nearby surroundings will reach the camera after the camera has been switched to an “on” state, and so light reflected towards the camera from the target will be fully collected by the camera.
The camera switches from an “off” state to an “on” state in a synchronized manner with the time required for the pulse to travel to the target and return. After the light reflected from the target has been received and stored, the camera reverts to an “off” state, and the system awaits transmission of the following light pulse. This cycle is repeated at a rate established in accordance with the range from the camera to the target, the speed of light in the observation medium, and the inherent limitations of the laser device and the camera. This technique is known as gated imaging with active illumination to minimize backscatter.
U.S. Pat. No. 5,408,541 to Sewell entitled “Method and System for Recognizing Targets at Long Ranges”, is directed to a method and system for recognizing targets at ranges near or equal to ranges at which they are initially detected. A detect sensor, such as a radar system or a thermal imaging sensor, detects a target relative to a sensor platform. The detect sensor determines a set of range parameters, such as target coordinates from the sensor platform to the target. The detect sensor transfers the set of range parameters to a laser-aided image recognition sensor (LAIRS). The LAIRS uses the set of range parameters to orient the system to the angular location of the target. A laser source illuminates the area associated with the range parameters with an imaging laser pulse to generate reflected energy from the target. A gated television sensor receives the reflected energy from the illuminated target, and highly magnifies and images the reflected energy. The image is then recognized by either using an automatic target recognition system, displaying the image for operator recognition, or both.
It is noted that Sewell requires a preliminary range measurement. Before the laser source illuminates the target, the laser source directs a low power measurement laser pulse toward the target to measure the range between the system and the target. The range sets a gating signal for the gated television sensor. The gated television sensor is gated to turn on only when energy is reflected from the target. It is also noted that the measuring line to the target of the laser ranger must be parallel, in a very accurate manner, to the LOS of the observation system.
It is an object of the disclosed technique to provide a novel system and method for gated imaging using active illumination that does not require a preliminary range measurement. It is a further object of the disclosed technique to provide for target identification in the FOV of a camera from a minimal range.